CREATIVE POWER · LEARNING HUB

One pair of wires,
talks to the whole plant

RS485 training with RS, our mascot
Recommended cable, installation, cautions and Modbus / Profibus frame examples

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RS485 · 01 / 09

Meet RS485 + its history

A differential digital signal on a pair: A white · B black

I'm RS485, a two-wire digital link
Born in 1983 at the EIA → standard TIA/EIA-485-A (1998)
Data is sent as the voltage difference between A and B

1983EIA publishes RS-485

1998TIA/EIA-485-A

TodayModbus · Profibus · meters · PLCs

RS485 · 02 / 09

Recommended cable

18AWG twisted pair + shield · same spec as Foundation Fieldbus Type A cable

18AWG (≈0.8 mm²) twisted pair + shield
Same spec as Foundation Fieldbus Type A: 100 Ω at 31.25 kHz
A = white · B = black · ends fitted with cord ends

18AWG ≈ 0.8 mm²Twisted pair + ShieldFoundation Fieldbus Type A100 Ω ±20% @ 31.25 kHz

A = whiteB = blackCord ends on every wire end

RS485 · 03 / 09

Why RS485

Noise-resistant · long distance · many devices on one pair

Differential: noise hits both wires equally → it cancels out
Distance up to about 1,200 m (at low speed)
Multi-drop: 32 unit loads per segment on 2 wires

~1,200 mat low speed

32unit loads / segment

2wires, many devices

RS485 · 04 / 09

Installation

Daisy chain · 120 Ω at both ends · shield grounded at one point

Run a continuous daisy chain; no star branches
Fit a 120 Ω resistor at both ends of the line
Ground the shield at one end · bias at one point
Add a repeater at 30 devices or 600 m

✓ daisy chain✗ No star branchesRepeater every 30 devices / 600 m · ≥ 0.1 mm/V from power cables

RS485 · 05 / 09

Cautions

Most problems come from wiring and settings

A/B pin names differ between brands; check the manual first
Same baud rate / parity on the whole line · no duplicate IDs
Keep away from motor / inverter cables · surge protection outdoors

⇄A/B pin names differ between brandsSome call them D+ / D− or swap A/B · check the manual before wiring

⚙Match settings on the whole lineSame baud rate · parity · stop bit · no duplicate Slave IDs

⚡Noise / surgesKeep away from motor / inverter cables · use a surge protector outdoors

RS485 · 06 / 09

Who shares our medium

Modbus · Profibus DP · Foundation Fieldbus

Modbus RTU: runs straight on RS485 · most common in meters and PLCs
Profibus DP: uses RS485 · 9.6 kbit/s – 12 Mbit/s · its own purple 150 Ω cable
Foundation Fieldbus H1: same cable type, but IEC 61158-2 signalling · cannot share a line

System	Physical layer	Speed	Cable	RS485?
Modbus RTU	RS485	1.2–115.2 kbit/s	STP 18AWG (ours)	✓ RS485
Profibus DP	RS485	9.6 kbit/s – 12 Mbit/s	Type A, purple, 150 Ω	✓ RS485
Foundation Fieldbus H1	IEC 61158-2	31.25 kbit/s + power on the bus	STP 18AWG Type A (same type)	✗ Different signalling

The same cable type works, but never put Foundation Fieldbus on the same line as RS485

RS485 · 07 / 09

Modbus example

Read 2 registers from device number 1

Master sends: 01 03 00 00 00 02 C4 0B
ID · function 03 · address · count · CRC
Slave replies: 01 03 04 00 0A 01 F4 DA 26 = values 10 and 500

Master →

01ID03Function 030000Address0002CountC40BCRC-16

03 = Read Holding Registers · CRC sent low byte first

← Slave

01ID03Function04Data bytes000A= 1001F4= 500DA26CRC-16

RS485 · 08 / 09

Profibus example

SD1 and SD2 frames · checked with the FCS sum

SD1 status request: 10 05 02 49 50 16
SD2 data exchange: 68 05 05 68 05 02 7D 12 34 CA 16
FCS = sum of the bytes from DA to the data (kept to 1 byte) · ends with 16

SD1

10SD105DA destination02SA source49FC status50FCS16ED

SD2

68SD20505LE LEr68SD20502DA SA7DFC1234DataCAFCS16ED

FCS = (DA + SA + FC + data) mod 256 · every frame ends with 16 (ED)

RS485 · 09 / 09

How to use it

Plan → configure → wire → 120 Ω → test a reading

Plan device count, distance and repeater points
Set ID / baud rate / parity on every device
Wire A white B black as a daisy chain + 120 Ω at both ends
Test a reading through CPL Gateway or Universal Gateway

1Plan count / distance / repeaters

2Set ID · baud · parity

3A white · B black · daisy chain · 120 Ω

4Test a reading through the gateway

One pair of wires,talks to the whole plant